1. Technical Field
The present invention relates to the field of telephony networks in general and to the control of calls in telephony networks in particular.
2. Related Art
An increasing number of telephone subscribers have more than one telephony account, benefiting from the increased flexibility and improved accessibility that multiple accounts can provide. For example, a subscriber may have a first, Wi-Fi telephony account and a second, GSM telephony account (a list of acronyms is provided at the end of the description). In such a telephony system, the Wi-Fi account can often be out of operation either because the Wi-Fi handset is switched off or is out of range of the nearest Wi-Fi access point (or “hotspot”). It is, therefore, convenient for the user, when their Wi-Fi telephony account is unavailable, for calls directed to the user at their Wi-Fi account to be redirected to a second telephony account, e.g., one provided via the GSM networks. A problem arises in the case of such re-directed calls when the call is not answered at the handset operating on the second network to which the call has been redirected. The default behavior of a telephony system in this situation would be for the call to be forwarded to a messaging service, e.g.: voicemail service, associated with the handset in the second network, to which the call has been diverted. This behavior, however, can lead to confusion for the user with messages, which should be associated with, and directly accessible from, their Wi-Fi handset, being instead associated with their GSM handset. The default behavior can also result in extra cost to the user where the user is liable to pay the mobile operator for a second diversion from the mobile network back to the redirecting network. Similar problems can occur with other combinations of telephony accounts, such as GSM with PSTN, where calls to the user may be redirected to the PSTN, when the user's GSM handset is switched off or out of range of a GSM transmitter.
The behavior described above may be less of a problem where both handsets are provided by the same service provider. Published patent application US 2007/0070976 describes a telecommunication system in which a service provider provides both a mobile network and a VoIP network interlinked via a PSTN backbone. Both networks provide telephony services to a user. According to this telecommunication system, calls to the user directed to a terminal on the fixed network may be delivered to a terminal associated with that user on the mobile network. If the call is not completed to the second terminal on the mobile network, the mobile network is able to detect this and to return the call to the fixed network, where it is forwarded to a messaging service on the fixed network.
The telecommunication system of U.S. Pat. No. '976 should work for users who have dual accounts from the same service provider. For example, “TruPhone” from Software Cellular Network, London, England is a service that provides a software application for mobile phones. This application provides end-users with a second Voice-over IP account, which works alongside an existing mobile phone account. In practice, not all users will have dual accounts from the same service provider and not all networks will be set up like the second network of U.S. Pat. No. '976. In a situation where the user has a first account with a first service provider operating a first network and a second account with a second service provider operating a second network, the above system may not work if, as is likely, there is no overall, control mechanism that allows the second network to be aware that an incoming call has been redirected from another network or to be alert to the need to return the call, if unanswered, to a messaging service in the other network.